When simply looking at a galaxy's image, lenticular galaxies with relatively face-on disks are difficult to distinguish from ellipticals of type E0–E3, making the classification of many such galaxies uncertain. Unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended, disk-like structure. The Spindle Galaxy (NGC 5866), a lenticular galaxy with a prominent dust lane in the constellation of Draco.Īt the centre of the Hubble tuning fork, where the two spiral-galaxy branches and the elliptical branch join, lies an intermediate class of galaxies known as lenticulars and given the symbol S0.
Įxamples of elliptical galaxies: M49, M59, M60, M87, NGC 4125. Observations of the kinematics of early-type galaxies further confirmed this. However, from studying the light profiles and the ellipticity profiles, rather than just looking at the images, it was realised in the 1960s that the E5–E7 galaxies are probably misclassified lenticular galaxies with large-scale disks seen at various inclinations to our line-of-sight. Observationally, the most flattened "elliptical" galaxies have ellipticities e = 0.7 (denoted E7). It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or highly elliptical if viewed edge-on). The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. By convention, n is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as e = 1 − b / a for an ellipse with semi-major and semi-minor axes of lengths a and b respectively. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity in the sky. Elliptical galaxies have relatively smooth, featureless light distributions and appear as ellipses in photographic images. On the left (in the sense that the sequence is usually drawn) lie the ellipticals. The giant elliptical galaxy ESO 325-G004.
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